The invention relates to wireless communication systems and, more particularly, to a burst control methodology for code division multiple access (CDMA) systems.
Wireless communication systems have been developed to allow transmission of information signals between an originating location and a destination location. Both analog (first generation) and digital (second generation) systems have been used to transmit such information signals over communication channels linking the source and destination locations. Digital methods tend to afford several advantages relative to analog techniques, including, e.g., improved immunity to channel noise and interference, increased capacity, and improved security of communication through the use of encryption.
While first generation systems were primarily directed to voice communication, second generation systems support both voice and data applications. Numerous techniques are known in second-generation systems for handling data transmissions which have different transmission requirements. Several modulation/coding arrangements have been developed for wireless systems based on multiple access techniques, e.g., frequency division multiple access (FDMA), time division multiple access (TDMA) and code division multiple access (CDMA). In FDMA techniques, each user is allocated one or more specific sub-bands of frequency. In TDMA techniques, periodically recurring time slots are identified, and for each segment of time each user is allocated one or more time slots. CDMA systems provide reduced multiple path distortion and co-channel interference and reduce the burden of frequency/channel planning that is common with FDMA and TDMA systems.
In a CDMA system, a unique binary spreading sequence (a code) is assigned for each call to each user. Multiplied by the assigned code, the user""s signal is spread unto a channel bandwidth much wider than the user signal bandwidth. The ratio of the system channel bandwidth to the user""s bandwidth is commonly called the spreading gain. All active users share the same system channel bandwidth frequency spectrum at the same time. Calculating the signal-to-interference (S/I) ratio determines the connection quality of the transmission link. Given a required S/I ratio, the system capacity is proportional to the spreading gain. The signal of each user is separated from the others at the receiver by using a correlator keyed with the associated code sequence to de-spread the desired signal.
First-generation analog and second-generation digital systems were designed to support voice communication with limited data communication capabilities. Third-generation wireless systems, using wide-band multiple access technologies such as CDMA, are expected to effectively handle a large variety of services, such as voice, video, data and imaging. Among the features which will be supported by third-generation systems is the transmission of high-speed data between a mobile terminal and a land-line network. As is known, high-speed data communications is often characterized by a short transmission xe2x80x9cburstxe2x80x9d at a high data transmission rate, followed by some longer period of little or no transmission activity from the data source. To accommodate the bursty nature of such high-speed data services in third-generation systems, it is necessary for the communication system to assign a large bandwidth segment (corresponding to the high data rate) for the duration of the data burst from time to time. With the ability of the third generation systems to handle such bursty high-speed data transmission, throughput and delay for users can be advantageously improved. However, because of the large amount of instantaneous bandwidth required for transmission of a burst of high-speed data, the management of such bursts, and particularly the allocation of power and system resources thereto, must be handled with care to avoid unwarranted interference with other services using the same frequency allocation. Consequently, system designers need to deal with many issues in setting efficient data rates for different types of communications via a wireless link, including appropriate allocation of system resources for the bursts of data experienced with high-speed data service.
It is also well recognized that frequency spectrum is a scarce resource, and wireless communication systems are allocated a fixed, and relatively limited portion of that spectrum. Thus, an important objective in wireless system design is the achievement of high spectrum efficiency. For wireless systems serving both voice and data transmissions, there is also a need to maintain a signal-to-interference ratio (SIR) objective for each user, while, at the same time, increasing the number of users which can be concurrently supported by the system and/or accommodating higher data rates for some portion of the users.
Hence, there is a continuing need to increase the performance of communication systems by accommodating a variety of users with different data rates. Specifically, there is a need for a better burst control methodology that improves the performance of CDMA systems with high-speed data transmission services.
In addition, there is a need to increase the throughput and data rate of an individual wireless system user, particularly for high-speed data. Accordingly, there is a corollary need for a better methodology for monitoring and determining the transmission rate for data channels in such systems.
The invention provides a novel methodology for increasing the performance of CDMA systems with packet data services by accommodating a variety of users with different rates. The invention operates to increase the spectrum efficiency of a wireless system by using an optimal transmission set of base transceiver stations (BTSs) and assigning proper data rates to efficiently utilize the radio resources. The invention operates to decrease interference so that the quality of data transmission in CDMA systems is advantageously maintained. Moreover, the invention operates to increase the allocation of bandwidth, particularly for high-speed data services. The invention provides a methodology to evaluate the data rates of different combinations of BTSs that may be in connection with a particular mobile station (MS), allocate an appropriate data rate for the MS and make allocation of system resources more efficient in a multi-user environment.
The method of the invention first detects a relative signal strength received at a particular MS from the BTSs in the active set, or the set of BTSs that may be in connection with the MS. Reduced active sets, or subsets, of the active set of BTSs serving the MS are created. The subsets are possible combinations of BTSs in the active set that serve the MS. The geometry of each subset is then determinedxe2x80x94that geometry being related to the signal-to-noise ratio (SIR) of the data channels connecting the BTSs in a subset with the MS. The power adjustment, or power per bit difference, is then calculated for each subset using the geometry. The affordable data rate is then determined for each subset, using the power adjustment and available power of the BTSs in the subset. The minimum of the data rates of all of the BTSs in a subset is chosen as the affordable data rate for that particular subset. The maximum of the affordable data rates of all the subsets is selected as the data rate for the MS.